The invention relates to a static trip device of a protective circuit breaker of an A.C. electrical power system made up of at least two conductors, trip device comprising detection means of the phase currents flowing in the power system conductors, detection means of the earth fault current flowing in the power system, an electronic processing unit to which the output signals from the detection means are applied and which performs an earth fault protection function by generating, with or without a time delay, a circuit breaker tripping order in the event of a preset differential tripping threshold being exceeded by the earth fault current.
When the phase currents in the power system are higher than a certain threshold, the vector sum of the currents present on the current transformer secondaries is not nil even in the absence of an earth fault. A signal, called false earth fault current, is then detected by the detection means of the earth fault current on which it is superposed. In order to avoid spurious circuit breaker trips, due not to an earth fault but to the existence of this false earth fault current, some known circuit breakers, performing an earth fault protection function, comprise an earth fault protection desensitization system. In being the rated current of the circuit breaker, state-of-the-art desensitization systems enable the earth fault tripping threshold to be raised when one of the phase currents exceeds a first preset value. As represented in FIG. 1, the earth fault tripping threshold Is is constant, for example equal to 0.2 In, when the phase current Ip is lower than 2 In, then this threshold Is increases linearly with the phase current when the latter is greater than 2 In. For very high phase current values, this desensitization is generally not sufficient, the false earth fault current then increasing very quickly, and it has been proposed to inhibit the earth fault protection when the phase current reaches a second preset value, for example 6 In as represented in FIG. 1. The values mentioned above are naturally given as an indication only.